Using regular laundry detergent in your high efficiency washing machine will not harm your washing machine or your clothes but what will happen if you use regular laundry detergent in your high efficiency washing machine is that your clothes may not get as clean since the regular laundry detergent will create too many soap suds and not rinse out of the clothes properly.
When the suds can't be rinsed out of the clothes properly the dirt and other soils in the clothes will not be cleaned completely out of the clothes.
Laundry detergent made for high efficiency washing machines is made so it doesn't make as many suds which will rinse out of the clothes and other laundry properly.
That's why it's important to use the correct laundry detergent in a high efficiency washing machine so your washing machine will work properly and get your clothes clean like it should.